It sounds to me that you are not quite clear on what the difference is between an IPO, stocks, and stock options.
I am not a certified financial adviser, but here is a VERY rough overview of them.
Stocks. Stocks are shares of a company. They document a percentage of ownership of the incorporated entity. Shares can be either publicly or privately traded. Privately held companies' shares are usually traded among a very small number of people. Usually the partners who formed the company and some "angel investors" who funded the initial version of the company. Privately held companies' share-trades are not regulated by the Securities and Exchange Commission, but are still regulated by state and federal regulations.
Stock Options. Options are an offer to sell a specified range of shares at a certain price on or before a certain date. They can be valuable if the stock's price rises higher than expected. If you are offered an option to buy 100 shares at $10 each on or before January 1st of 2015, and when that date comes the shares are worth $20 each, you can buy the 100 shares for $10 per and sell for $20 per, netting $1,000 (minus trading fees, of course). If the stock is only worth $5 on 1/1/2015, you are not required to purchase at $10 per, thus the term "Option."
IPO - Initial public offering. A company is valued by underwriting financial institutions and certified to the Securities and Exchange Commission. Shares are issued based on the percentage of the company that is being "Offered" for public trading. As of that moment, all significant financial dealings and all intended and actual share trading by senior management and board members are required to be reported to the Securities and Exchange Commission quarterly, annually, and a whole new regulatory environment is entered.
When a company "Goes IPO," employees are often given the opportunity to buy a limited number of shares at the initial offer price. They are sometimes given the opportunity to buy at that price for several months after the IPO in the form of stock options. The reason for this is that it's actually quite difficult to buy a stock on its IPO. You have to be well-connected. Usually you will end up buying them after they've traded hands through a brokerage or two. The "buzz" around IPO's is that if investors feel a company was undervalued by the underwriters of the IPO, the stock will immediately go up. The $10/share IPO may be trading at $11.50 later that day, and whoever got the $10 shares makes a good profit. It's hard to be in that group, so that's why employees are sometimes given a chance to "cut in line" and get the IPO price for a limited number of shares.
Now unless you seriously know what you're doing in the markets, you really don't want to try to time your transactions by buying at pre-IPO and selling a few days later. Also, if 500 employees each got 5000 IPO stock options, and they all buy them on Monday and sell them on Thursday, that can seriously distort the stock's trading performance. That's why there's usually a restriction on how long you have to hold pre-IPO purchased shares before you are allowed to sell them.
Now as verbose as the above is, it represents a tiny amount of the information you need to know about investing in the company you work with. I strongly recommend consulting with a licensed broker or certified financial planner before making any decisions. However, be aware of your Non-Disclosure requirements with your company at the same time. If you tell a broker, "My company's doing an IPO next year, and I wonder if ...." Guess what the broker heard? "Company XYZ is going IPO next year." He's going to use that. Make sure you're not violating any NDA's.